ATLANTA (Reuters) - Former pizza magnate Herman Cain dropped out of the U.S. presidential race on Saturday after accusations of sexual misconduct overwhelmed his bid to win the Republican nomination as an anti-Washington tax reformer.

Cain's departure set off a competition among other candidates to win over his conservative supporters with voting to start next month in the race to determine the Republicans' presidential pick for 2012 to oppose President Barack Obama.

Cain, a former Godfather's Pizza chief executive, told followers in his hometown of Atlanta that "false and unproved" sexual accusations had forced him to suspend his White House bid.

He said he would endorse another candidate, effectively ending a roller-coaster campaign that saw the African-American businessman go from complete outsider to the rising star of Republican conservatives within a few weeks over the autumn.

Cain held a Friday night meeting with his wife, Gloria, who has stayed mostly silent as several women accused her husband of sexual harassment and an Atlanta businesswoman this week said she had carried out a 13-year affair with him.

Gloria Cain appeared at his side on Saturday and smiled throughout.

Cain's departure shapes the wide-open Republican race more clearly into a matchup between former Massachusetts Governor Mitt Romney and surging rival Newt Gingrich, former speaker of the U.S. House of Representatives.

Reuters/Ipsos poll data released this week showed Gingrich and Romney would benefit most if Cain quit.

Support for Romney and Gingrich would rise by 3 percentage points each, against the 1- or 2-point bumps Cain's departure would give some of their rivals for the nomination.

Gingrich, a fellow Georgian, said he would meet with Cain next week.

"I don't anticipate anything," CNN reported Gingrich as saying. Cain has to "make up his own mind," Gingrich said.

Romney said in New Hampshire he sought backing from Cain supporters. "I hope as they evaluate the various candidates, they conclude that I'm the leader they need," he said.

Suspending his campaign instead of quitting altogether lets Cain continue raising money that he could use to pay off any debt he had run up.

Republican presidential candidate businessman Herman Cain gestures at the crowd as his wife Gloria looks on during his announcement that he is "suspending" his campaign at his Georgia campaign headquarters in Atlanta, Georgia December 3, 2011.John Adkisson

CLOUD OF DOUBT

Cain, 65, said a "cloud of doubt" had settled over his campaign. He insisted that accusations he sexually harassed several women and had an affair with Atlanta businesswoman Ginger White were simply not true.

"I am suspending my campaign because of the continued distraction, the continued hurt," Cain said at an event that had been planned as the grand opening of his campaign headquarters in Atlanta.

Cain won attention for his 9-9-9 plan to reduce the U.S. tax code to 9 percent on income, sales and corporate taxes.

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After jumping into the front-runner position two months ago, Cain had been in a free fall in the past month. His support in Iowa, which holds the first U.S. nominating contest on January 3, stands at 8 percent.

Cain spoke of having made mistakes but did not elaborate. His campaign also suffered from several stumbles that included a long pause when he tried to answer a routine question about Libya.

"I've made mistakes professionally, personally, as a candidate, in terms of how I run my campaign. And I take responsibility for the mistakes that I've made. And I've been the very first to own up to any mistakes I've made," Cain said.